ABSTRACT Our investigative team has been studying the intricacies of self-monitoring for over a decade and has documented that consistency and timing of self-monitoring in relation to eating is significantly related to weight loss outcomes. More recently, evidence is building to support adding feedback (FB) messages to self- monitoring for reinforcing behavior change and enhancing motivation and adherence. Empirical evidence demonstrates that the more proximal the reinforcing message is delivered following the performance of the desired behavior, the more likely it will increase the desired behavior. The proposed study is based on this body of evidence, in part provided by a previous trial conducted by our team and promising pilot data of the proposed intervention. Subjects in the previous weight loss trial were more adherent to self-monitoring and the treatment protocol and more likely to achieve 5% weight loss if they received a once-daily FB message tailored to what they had recorded that day than those who did not receive FB. Building on this study, we have developed an extensive FB algorithm with more than 1000 conditions to address diet and physical activity. Given our findings from our past research on self-monitoring in weight loss and the results of our pilot feasibility study testing the FB in conjunction with SM, we propose to conduct a full-scale, 4-year RCT where 530 adults will be randomized 1:1 to either: 1) self-monitoring (SM), where subjects will self-monitor diet, physical activity and weight using a commercially available self-monitoring application; or 2) SM+FB, where participants self- monitor and receive real-time FB messages. The group with FB will receive up to 4 daily pop-up FB messages on their smartphone delivered randomly during waking hours and tailored to entries in the subjects? smartphone-based diaries. The primary specific aims for this study are to: 1) determine if SM with tailored FB delivered in real time up to 4 times per day is superior for promoting weight loss at 6 and 12 months to the SM condition and 2) Compare sustainability of engagement between the SM and the SM+FB groups for the full 12- mo study as measured by adherence to components of study protocol at 3, 6, 9 and 12 mos. We will also conduct innovative analyses to explore the temporal relationships of the frequency, timing and type of FB delivered and subsequent lifestyle behaviors through examination of serially collected real-time SM data. The proposed research addresses a major public health problem and is innovative in its large scale testing of a novel FB algorithm and program to reinforce healthful behaviors and its examination of the sustainability of engagement. If proven efficacious, this intervention could be efficiently translated and through integration with commercial apps could be fully scalable and disseminated to reach large numbers of individuals who wish to improve their health with lower in cost than existing programs.